Method and apparatus for selection of perspective orientation of a two dimensional angiographic image

ABSTRACT

Systems and methods provide guidance for selection of projection perspectives to utilize to obtain complementary combinations of projection images of an object. The systems and methods provide a first two-dimensional image of the object which has been obtained from a first perspective having a first spatial orientation with reference to a coordinate system. The systems and methods define a first parameter indicative of a degree to which candidate perspectives complement the first perspective, define at least one scale of values between first and second limits, the first and second limits are associated with first and second candidate perspectives having complementary and non-complementary relations to the first perspective. The systems and methods associate the values to the first parameter for the candidate perspectives and display indicia indicative of the values of the first parameter with reference to the coordinate system as guidance for selecting candidate perspectives.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from EP Patent Application No.EP16155314.4, filed on Feb. 11, 2016, herein incorporated by referencein its entirety.

BACKGROUND

1. Field

The embodiments herein relate to a method and apparatus for determiningoptimal projection images of an object of interest, particularlyangiographic images.

2. State of the Art

X-ray angiography is a commonly used imaging modality within a numerousvariety of interventions. During the interventions, it is very importantthat the clinician achieves a good understanding of the object inquestion using a workflow that is as efficient as possible. That is, amethod that is fast, reproducible and burdens the patient minimally.

During X-ray angiography several different two-dimensional images, alsocalled two-dimensional projections, of the object under examination canbe obtained from different views or perspectives by rotating the arm,holding the X-ray source and the image intensifier, with reference tothe patient.

It is common practice to use two acquired two-dimensional angiographicimages to generate a three-dimensional reconstruction for example a partof the vascular system. This three-dimensional reconstruction is thenthe basis for performing 3D quantitative analysis on (part of) a vesselof interest or for instance to perform computational fluid dynamicsimulations.

Because the three-dimensional reconstruction is the basis for furthercalculations, it is important that the three-dimensional reconstructionis as accurate as possible. The choice of the two two-dimensionalangiographic images is determinative for the accuracy of thethree-dimensional reconstruction.

This is due to different aspects. First of all, the two two-dimensionalimages used to generate the three-dimensional reconstruction shouldcontain as much information regarding the object of interest aspossible.

Furthermore, the accuracy of the three-dimensional reconstruction is notsolely dependent on the amount of information that is present in the twotwo-dimensional angiographic images, as the spatial angle between thetwo images is of importance. When the spatial angle between the twotwo-dimensional angiographic images is too small, the geometry of thevessel is unclear because the images contain roughly the sameinformation regarding the object of interest.

At the moment, several methods have been proposed to determine optimalviews or perspectives at which the clinician should acquiretwo-dimensional angiographic images to allow accurate 3D reconstruction.These optimal views are however determined using 3D information of theobject of interest as described for instance in U.S. Pat. No. 9,129,418.In practice this means that a clinician acquires two two-dimensionalangiographic images. These images are then used to generate athree-dimensional reconstruction which is subsequently used to determinethe optimal perspective(s). It is not until after generating thethree-dimensional reconstruction that the clinician obtains informationon how optimal the two two-dimensional angiographic images are that wereused to generate the three-dimensional reconstruction. If the used twotwo-dimensional images were not optimal, the clinician has to acquire anew two two-dimensional angiographic image and generate a newthree-dimensional reconstruction or at least one according to theteachings of European patent application published with numberEP2570079.

A large disadvantage of these approaches is that a 3D reconstruction ofthe object of interest is required to determine if the usedtwo-dimensional projections are optimal. Because of this, a completeanalysis of the images has to be performed before the initially chosenimage projections can potentially be replaced with more optimalprojections. This is time consuming and poses a burden on the patient interms of more contrast fluid as well as more exposure to x-rayradiation.

There is therefore a need for a more efficient approach that optimizesthe workflow for the clinician and reduces the burden to the patient.

SUMMARY

It is thus an object of the embodiments herein to provide a method toefficiently guide a user in selecting the optimal projection of atwo-dimensional (two-dimensional) angiographic image before generating athree-dimensional reconstruction of the object of interest. This savestime, effort and is of less burden for the patient because there is noneed to acquire an additional angiographic image that is more optimal.

In accordance with embodiments herein, systems, computer programproducts and computer implemented methods are provided for providingguidance for selection of projection perspectives to obtaincomplementary combinations of projection images of an object. Thesystems, program products and methods operate, under control of one ormore computer systems configured with specific executable instructions,to:

-   -   a) provide a first two-dimensional image of the object which has        been obtained from a first perspective having a first spatial        orientation with reference to a coordinate system;    -   b) define a first parameter indicative of a degree to which        candidate perspectives complement the first perspective;    -   c) define at least one scale of values between first and second        limits, the first limit associated with a first candidate        perspective having a complementary relation to the first        perspective, the second limit associated with a second candidate        perspective having a non-complementary relation to the first        perspective;    -   d) associate the values to the first parameter for the candidate        perspectives; and    -   e) display indicia indicative of the values of the first        parameter with reference to the coordinate system as guidance        for selecting one or more candidate perspectives to utilize to        obtain a combination of complementary projection images.

Optionally, candidate perspectives can be identified by coordinates, thefirst parameter being displayed in the form of a map where the indiciarepresent a color or grey value that is associated with combinations ofthe coordinates. Optionally, the first spatial orientation of the firstperspective and a spatial orientation of the candidate perspectives areboth expressed in the coordinate system and in a form of rotation andangulation of an x-ray machine that is configured to obtain the firsttwo-dimensional image and at least one second two-dimensional image.

In accordance with embodiments herein, the systems, program products andmethods further include calculating spatial angles between the firstperspective and the corresponding candidate perspectives; associatingeach of the spatial angles with a value for the first parameter;defining a second parameter; projecting each of the candidateperspectives on the first image to obtain a set of candidate projectedperspectives; determining a differential difference angle between eachof the candidate projected perspectives and a reference line located onthe first two-dimensional image; associating each of the differentialangles with a value for the second parameter; combining the spatialangles and the differential angles for the corresponding candidateprojected perspectives to form an output parameter; and displayingindicia indicative of a value of the output parameter with reference tothe coordinate system as guidance for the selection of a desired secondperspective.

Optionally, the projected perspectives are epipolar lines correspondingto each candidate perspective as projected on the first image. The organmay represent a tubular organ or comprises a region containing tubularorgans, the reference line being the centerline of the tubular organ ororgans. The organ may comprise a plurality of tubular organs, thereference line is the centerline of at least part of the tubular organs,a weighting function being defined to weigh the contribution of eachorgan to determine an average differential difference angle. The firstparameter can correspond to spatial angles and the second limit can beone of i) at or less than 30° or ii) at or higher than 150°. The organcan comprise a plurality of organs, where the method further providesfor inputting a 3D or 3D+t model of the organ to be used for associatingvalues to the first parameter as a function of the degree of overlapbetween organs when second perspectives of the set are used to obtaincorresponding projection images. The candidate perspectives can containorgan overlap with the first perspective are assigned least optimalprojection values for the first parameter, while the candidateperspectives containing little or no overlap with the first perspectiveare assigned a most optimal projection value for the first parameter.The first parameter can represent at least one of a spatial angleparameter, a differential difference angle parameter and an overlapparameter. The angulation and rotation angles of the candidateperspectives are limited within a range of possible rotation andangulation angles of an apparatus used for acquiring the candidateprojections.

In accordance with aspects herein, a method is provided for guidance forthe choice of projection perspectives to obtain optimal projectionimages of an object (particularly an asymmetrical object), the methodincluding:

-   -   a) providing a two-dimensional image of the object which have        been obtained from a first perspective having a first spatial        orientation with reference to a system of coordinates;    -   b) defining a set of second perspectives;    -   c) defining a first parameter;    -   d) defining at least one scale of values between a minimum and a        maximum for the first parameter with the maximum value being        associated to the most optimal perspective and the minimum to        the least optimal perspective or vice versa;    -   e) associating a value to the first parameter on such a scale        for each of the second perspectives; and    -   f) displaying the value of the first parameter with reference to        the system of coordinates as guidance for the choice of an        optimal perspective.

By suggesting to the user the two-dimensional angiographic image(s)which is (are) most optimal to be used with a first two-dimensionalangiographic image, for example to allow further analysis such asthree-dimensional reconstruction or computational fluid dynamics, theworkflow for the clinician is optimized, particularly by providing anoptimal projection map that contains for each possible angiographicprojection a measure for how optimal that projection would be as asecond projection.

The clinician can then decide which of the proposed optimal projectionsis best suitable for the current procedure. For instance, for someprocedures certain projections are less favorable due to the practicalapplicability of the system angles. All that without the need to makeany preliminary three-dimensional reconstruction.

Furthermore, to obtain a good understanding of the object of interest itis important that the clinician has a clear view of the object ofinterest. That is, a view that is minimally obstructed for instance dueto overlap of surrounding vessels. In an X-ray angiographic imageoverlapping vessels cannot be distinguished due to superimposing and aretherefore extremely cumbersome.

To an extent, embodiments can also take into account overlap ofsurrounding vessels.

Advantageously second perspectives can be identified by coordinates, thefirst parameter being displayed in the form of a map where a color orgrey value is associated to combinations of the coordinates. Embodimentsprovide that the first spatial orientation of the first perspective isexpressed in the form of rotation and angulation of the x-ray machineused for obtaining the first two-dimensional image and the spatialorientation of the second perspectives are expressed in the form ofrotation and angulation angles in the same reference system. Angulationand rotation angles of the second perspective may be advantageouslylimited within a range of possible rotation and angulation angles of theapparatus used for acquiring projections according to the perspectives.

According to an embodiment, the method further includes:

-   -   g) calculating the spatial angle between each perspective of the        set and the first perspective;    -   h) associating to each spatial angle a value to the first        parameter;    -   i) defining a second parameter;    -   j) projecting each perspective of the set on the image to obtain        a set of projected perspectives (for example, epipolar lines        corresponding to each perspective of the set as projected on the        image);    -   k) determining a differential difference angle between each of        the projected perspectives and a reference line located on the        image, for example a centerline of a tubular organ (note that if        the organ comprises a plurality of tubular organs, the reference        line may advantageously be the centerline of at least part of        the tubular organs; in this case a weighting function may be        defined to weigh the contribution of each organ to determine an        average differential difference angle);    -   l) associating to each differential angle a value to the second        parameter;    -   m) combining the spatial angle and the differential angle in an        output parameter; and    -   n) displaying the value of the output parameter with reference        to the system of coordinates as guidance for the choice of an        optimal perspective.

The second parameter may be expressed in the same scale of values of thefirst parameter or a further scale of values between a minimum and amaximum may be defined for the second parameter. The maximum value isassociated to the most optimal perspective and the minimum to the leastoptimal perspective or vice versa. For example, least optimal projectionvalues for the first parameter are assigned to spatial angles less than30° and higher than 150°, preferably less than 20° and higher than 160°,more preferably less than 15° and higher than 175° while least optimalprojection values for the second parameter are assigned to differentialangles less than 90° with 90° corresponding to the most optimalprojection value and 0° to the least optimal projection value.

In one embodiment, the organ comprises a plurality of organs, and themethod further builds or inputs a 3D or 3D+t model of the organ to beused for associating values to the first parameter as a function of thedegree of overlap between organs when second perspectives of the set areused to obtain corresponding projection images. Perspectives containingorgan overlap are, for example, assigned least optimal values with nooverlap corresponding to the most optimal projection value of theparameter.

A registration may be advantageously performed between thetwo-dimensional mage and the three-dimensional model before assigning avalue to the parameter. If the three-dimensional model is a 3D+t model,an embodiment provides for synchronizing two-dimensional angiographicimages containing multiple heart phases with the heart phase using anECG signal before registration.

All parameters defined above can be used alone or jointly in anycombination between them. In an embodiment, a spatial angle parameter, adifferential difference angle parameter and an overlap parameter arecombined to obtain an output parameter to be displayed as guidance forthe choice of an optimal perspective.

The method is typically performed by a data processing system withaccess to two-dimensional images of an object of interest obtained fromdifferent perspectives.

Embodiments herein also relates to a computer product directly loadableinto the memory of a computer and comprising software code portions forperforming the method as disclosed above when the product is run on acomputer.

According to another aspect, embodiments herein also relate to anapparatus for acquiring two-dimensional projection images of athree-dimensional object. The apparatus comprises a data processingmodule programmed for performing the method according to the embodimentsherein to determine perspectives for obtaining optimal projection imagesof the object.

Advantageously, the apparatus could be the same machine used foracquiring and/or reconstructing the image data. Particularly it is anangiographic apparatus of the monoplane, biplane, C-arm or L-arm typewith X-ray source and image intensifier respectively located at oppositesides of the arm, the arm being movable at least according to a rotationangle and an angulation angle with reference to a patient to obtaintwo-dimensional images from different perspectives, the processingmodule being programmed to calculate rotation and angulation angles ofthe arm for obtaining optimal projection images.

According to an embodiment, the angiographic apparatus comprisesactuating module to automatically or semi-automatically rotate the arm,and/or display module for providing to a user indications for manuallyrotating the arm, according to rotation and angulation angles calculatedfor obtaining an optimal projection image. Advantageously, among twoperspectives having opposite viewing direction and therefore resultingin the same angiographic image, the processing means is programmed toselect the one corresponding to a rotation and angulation angle withinthe range of possible rotation and angulation angles of the apparatus.

The processing module could be a processor or processors dedicated toperform the method described herein or, in a particularly advantageousconfiguration, the same, or part of the same, processing module thatsubtends the main image acquisition functionalities of the machine thusobtaining a very compact and powerful apparatus.

Further improvements of the present application will form the subject ofthe dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics of the embodiments herein and the advantages derivedtherefrom will be more apparent from the following description ofnon-limiting embodiments, illustrated in the annexed drawings.

FIG. 1 shows a flowchart of a method for providing guidance forselecting projection perspectives performed in accordance with anembodiment.

FIG. 2 shows a flowchart of a method for providing guidance forselecting projection perspectives in accordance with an alternativeembodiment.

FIG. 3a shows the rotation and angulation movement of an X-ray systemwith indicated a reference system, the planes containing projection Iand projection J and the corresponding normal vectors identifying theperspectives or viewing directions IVD, JVD from which the projectionsare taken.

FIG. 3b shows the spatial angle between the viewing direction IVD ofprojection I and JVD of projection J.

FIG. 4 shows an example of an optimal projection map based on spatialangle where the value 1 is most optimal.

FIG. 5 shows an example of the epipolar lines and the direction vectorsfor the centerline points.

FIG. 6 shows an example of an optimal projection map based ondirectional difference angle where the value 1 is most optimal.

FIG. 7 shows an example of an optimal projection map based on spatialangle and directional difference angle where the value 1 is mostoptimal.

FIG. 8 shows a flowchart to obtain the overlap parameter.

FIG. 9 shows an example of x-ray cinefluorographic unit block diagram inaccordance with an embodiment herein.

FIG. 10 shows a functional block diagram of an exemplary single planeangiographic system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 shows a flow chart illustrating the operations according to anembodiment of the present application. The operations employ an imagingsystem capable of acquiring and processing two-dimensional images of avessel organ (or portion thereof) or other object of interest. Forexample, a single plane or bi-plane angiographic system can be used asthose manufactured, for example, by Siemens (Artis zee Biplane) orPhilips (Allura Xper FD).

FIG. 10 is a functional block diagram of an exemplary single planeangiographic system, which includes an angiographic imaging apparatus112 that operates under commands from user interface module 116 and willprovide data to data processing module 114. The single planeangiographic imaging apparatus 112 captures a two-dimensional X-rayimage of the vessel organ of interest for example in thepostero-anterior (PA) direction. The single plane angiographic imagingapparatus 112 typically includes an X-ray source and detector pairmounted on an arm of a supporting gantry. The gantry provides forpositioning the arm of the X-ray source and detector at various angleswith respect to a patient who is supported on a table between the X-raysource and detector. The data processing module 114 may be realized by apersonal computer, workstation or other computer processing system. Thedata processing module 114 includes one or more processors and memorythat stores program instructions to direct the one or more processors toperform the operations described herein. The data processing module 114also includes a display to present information to a user, such as theimages, indicia, data and other information described herein andillustrated in the figures. The data processing module 114 also includesa user interface to receive inputs from the user in connection withoperations herein, such as controlling operation of the imagingapparatus 112, selecting projection perspectives to be used whenobtaining complementary images and the like. The data processing module114 may correspond to or include portions of one or more of the systemsdescribed within the patents and publications referenced herein andincorporated by reference. The data processing module 114 processes thetwo-dimensional image captured by the single plane angiographic imagingapparatus 112 to generate data as described herein. The user interfacemodule 116 interacts with the user and communicates with the dataprocessing module 114. The user interface module 116 can includedifferent kinds of input and output devices, such as a display screenfor visual output, a touch screen for touch input, a mouse pointer orother pointing device for input, a microphone for speech input, aspeaker for audio output, a keyboard and/or keypad for input, etc. Thedata processing module 114 and the user interface module 116 cooperateto carry out the operations of FIG. 1 as described below. The operationsof FIG. 1 can also be carried out by software code that is embodied in acomputer product (for example, an optical disc or other form ofpersistent memory for instance an USB drive or a network server). Thesoftware code can be directly loadable into the memory of a dataprocessing system for carrying out the operations of FIG. 1.

In this example, it is assumed that the imaging system has acquired andstored at least one two-dimensional image (referred to herein as“projection image”) of an object of interest according to a perspectiveIVD. Any image device capable of providing two-dimensional angiographicimages can be used for this purpose. For example, a bi-plane or singleplane angiographic system can be used as those manufactured, forexample, by Siemens (Artis zee Biplane) or Philips (Allura Xper FD).

In step 10, the data processing module 114 is fed by a two-dimensionalimage I of the object which have been obtained from a first perspectiveIVD having a first spatial orientation with reference to a system ofcoordinates. For example, a processor of the data processing module 114may access the imaging apparatus 112 to obtain the two-dimensional imageI in real time while the imaging apparatus 112 is performing a scan of apatient. Optionally, the processor of the data processing module 114 mayaccess memory to obtain one or more pre-recorded two-dimensional imagesI. As a further example, the processor of the data processing module 114may access a database, server or other network accessible memory thatincludes prerecorded volumetric data sets for a patient. The image Iobtained at 10 is referred to throughout interchangeably as a firstimage or base image or primary image.

In step 12, one or more processors of the data processing module definesa set of second/candidate perspectives JVD. The set of second/candidateperspectives may be defined manually by the user. Additionally oralternatively, the one or more processors may define the set ofsecond/candidate perspectives automatically, such as based onpredetermined perspective definition criteria (e.g., angles). Optionallythe set of second/candidate perspectives may be automatically definedbased upon the type of scanned being performed or the nature of theanatomy/object being analyzed.

At 14 a first parameter is defined. This can be for example a spatialangle, a differential angle, an overlap value or other criteria asdefined and explained in details below with reference to the operationsassociated to FIGS. 2 and 8. The first parameter is indicative of adegree to which candidate perspectives complement a first perspective atwhich a first image was obtained. As explained herein, once a firstimage is obtained, alternative/candidate second images may be obtainedthat can be utilized with the first image in various manners, such as togenerate a three-dimensional reconstruction of an object of interest.Each candidate second image has a corresponding candidate perspectivealong which the second image is obtained. The various candidate secondimages (and corresponding various candidate perspectives) complementsthe first image two different degrees/amounts. The relation between thefirst image/perspective and the candidate perspectives may be defined invarious manners (e.g., spatial angle, differential angle, overlap valueand the like).

At 16, one or more processors of the data processing module 114 definesat least one range/scale of values associated with the first parameter.The scale of values ranges between first and second limits. The firstlimit is associated with a first candidate perspective having acomplementary relation to the first perspective of the first image,while the second limit is associated with a second candidate perspectivehaving a non-complementary relation to the first perspective of thefirst image. For example, the first and second limits may correspond toa minimum and a maximum for the first parameter with the maximum valuebeing associated to a complementary perspective (e.g. the most optimalperspective) and the minimum to a non-complementary perspective (theleast optimal perspective) or vice versa. If the first parameter is aspatial angle between the first perspective and the candidateperspectives, the non-complementary perspective (e.g. least optimalprojection) values for the first parameter may be, for example, assignedto angles at or less than 30° and at or higher than 150°, moreparticularly to angles at or less than 20° and at or higher than 160°.In case of the first parameter corresponding to a differential angle,the non-complementary perspective (e.g. optimal projection) values maybe assigned, for example, to differential angles less than 90° with 90°corresponding to the most optimal projection value and 0° to the leastoptimal projection value. If the parameter corresponds to an overlapvalue, candidate perspectives containing organ overlap with the firstperspective may be, for example, assigned non-complementary (e.g. leastoptimal) values, while candidate perspectives containing no overlap withthe first perspective may be assigned complementary (e.g. the mostoptimal projection) value of the parameter.

At 18, the one or more processors of the data processing module 114associates a value to the first parameter on the so defined arange/scale for each of the second/candidate perspectives. For example,when ten separate candidate perspectives are defined at 12, each of thecandidate perspectives is assigned a separate scale value for the firstparameter to indicate a degree to which the corresponding candidateperspective represents a complement or non-complement to the firstimage. Continuing with the above example, when the first parameterrepresents spatial angle, a candidate perspective having a spatial angleof 25° relative to the first perspective of the first image would beassigned a scale value indicative of a non-complementary relationbetween the candidate and first perspectives. Alternatively, when acandidate perspective has a spatial angle of 70° relative to the firstperspective of the first image, the candidate perspective may beassigned a scale value indicative of a complementary relation betweenthe candidate and first perspectives. As described herein, the scalevalues may be indicated through various types of indicia, such asnumerical values, bar charts, graphs, color-coding over a map,variations in grayscale over a map and the like.

At 20, the value of the first parameter with reference to the system ofcoordinates as guidance for the choice of an optimal perspective isshown on a display for the user to make the final choice/selection. Forexample, the display of the data processing module 114 may present a mapcontaining indicia indicative of the scale values for the firstparameter with reference to a coordinate system. The indicia affordguidance for the user to select one or more candidate perspectives toutilize to obtain one or more secondary images that, when joined withthe first image, provide a combination of complementary projectionimages.

In accordance with the process described in connection with FIG. 1, acomputer implemented method is provided for guidance for selection ofprojections perspectives in connection with obtaining one or moresecondary images that complement the first/primary image. Optionally, auser may select more than one secondary image. Optionally, once asecondary image/perspective is identified, the foregoing process may berepeated yet again utilizing the secondary image/perspective as a newfirst or primary image/perspective. When a new first or primary imageperspective is identified, the operations at 12-20 may be repeated toidentify a new or additional secondary images/perspectives in aniterative manner.

A further embodiment is now disclosed with reference to FIG. 2.

In this example, it is assumed to have at disposal a two-dimensionalangiographic image (I) of an object of interest (at 101). For example,the two-dimensional angiographic image data may be obtained in real timefrom an angiographic imaging system. Optionally, prerecordedtwo-dimensional angiographic image data may be obtained from a localmemory, a database, a network server or otherwise. This two-dimensionalangiographic image (I) may advantageously contain multiple framescovering multiple heart phases. Any image device capable of providingtwo-dimensional angiographic images can be used for this purpose. Forexample, a bi-plane or single plane angiographic system can be used suchas those manufactured, for example, by Siemens (Artis zee Biplane) orPhilips (Allura Xper FD). In the two-dimensional angiographic image, orin one of the frames of the two-dimensional angiographic image sequence,a centerline of a segment of interest is indicated as shown in step 102of FIG. 2. This can be done manually by the user or automatically bymethods well known in the art.

In order to be able to guide the user in the selection of the secondtwo-dimensional angiographic image, the processor of the data processingmodule 114 performs operations to determine a degree to which eachcandidate/possible projection J complements the first or primaryprojection I.

By way of example, the degree to which candidate projections Jcomplement the first/primary projection I is determined by calculatingtwo parameters for each candidate/possible projection.

At 103, the processor of the data processing module 114 calculates firstthe spatial angle between each candidate projection J and thefirst/primary projection I.

Each two-dimensional candidate projection J is usually associated to acertain rotation and angulation value identifying the orientation of theX-ray machine used for obtaining it and thus the perspective JVD fromwhich the projection is seen. In C-arm machines, the X-ray source isunder the table and the image intensifier is directly above the patient.The body surface of the patient that faces the image intensifier (orflat panel) determines the specific view. This relationship holds truewhether the patient is supine, standing, or rotated. To obtain anoblique (angulated from the perpendicular) view, the C-arm is rotatedsuch that the image intensifier is positioned toward the patient's right(RO—Right Anterior Oblique view) or left (LO—Left Anterior Oblique view)shoulder or toward the head (CR—Cranial view) or the feet (CA—Caudalview) as shown in FIG. 3a . The angles by which a left-right movement ofthe machine, with respect to the patient, can be defined are calledrotation angles. The angles by which a movement toward the head or thefeet of the patient can be defined are called angulation angles.

The perspective, or viewing direction, of the X-ray machine resultingfrom a particular projection I, J is dependent on the rotation andangulation angle of the C-arm and can be expressed as athree-dimensional (3D) unit vector IVD, JVD. When defining rotation as arotation around the x-axis and angulation as a rotation around they-axis of a 3D coordinate system as can be seen in FIG. 3a , thecoordinates (x, y, z) of the unit vector are:

This is done for each candidate/possible projection J as well as for thefirst/primary projection corresponds to I.

The spatial angle between a certain projection J and projection I is thethree-dimensional angle between the two corresponding viewing directionsJVD and IVD as can be seen in FIG. 3b . This three-dimensional angle maybe calculated, for example, using the dot product of the twocorresponding normal vectors divided by the Euclidian norm of thevectors, for example as follows.

An example of the outcome of the spatial angle determination between animage I and each candidate/possible other projections J can be seen inFIG. 4. In FIG. 4 an example of the outcome is shown as a color map. Thex-axis of FIG. 4 shows the rotation angles of the X-ray apparatus. They-axis of FIG. 4 shows the angulation angles of the X-ray apparatus. Forall combinations of rotation and angulation angles of the X-rayapparatus, that is all possible projections J, the spatial angle betweenprojection J and projection I is calculated as described above.

In this example, all candidate projections resulting in a spatial angleless than 30 degrees and larger than 150 degrees relative tofirst/primary projection I have been determined to be non-complementary(e.g. non-optimal), and are depicted black in the color map of FIG. 4.While all projections resulting in a spatial angle larger than 30degrees and smaller than 150 degrees are determined to be complementary(e.g. optimal) and are depicted white in the color map of FIG. 4.

At 104, the processor of the data processing module 114 calculates asecond parameter, e.g. the directional difference angle for eachcandidate/possible projection J.

For doing that, at 102 the processor firstly detects a reference line inthe two-dimensional angiographic image I. Any type of reference line canbe used for the purpose. In the example herein described the referenceline is a centerline of the object of interest, typically a vessel.

For each point of the reference line, the direction vector of the lineat that point is thus determined. In case of a centerline, this can bedone for instance by constructing a straight line between a centerlinepoint and the next centerline point.

Then the processor, for the current reference point, projects thecorresponding epipolar line corresponding to a particular secondprojection J onto the two-dimensional angiographic image as described byHan, “Contour matching using Epipolar Geometry”, IEEE Transactions onpattern analysis and machine intelligence, Vol. 22, No. 4 (2000), p358-370. Han teaches that between any two images (or, equivalent, anytwo camera systems) there is an epipolar geometry. Match candidatesbetween two images are established using a correlation-based technique.The complete subject matter of the publication referenced herein isincorporated by reference herein in its entirety.

At 104, the processor of the data processing module 114 determines thedirectional difference angle between the epipolar line per imageprojection J based on the rotation and the angulation and the directionvector of the reference line as can be seen in FIG. 5. Each referenceline point has a direction vector 501, from the current reference linepoint to the next and an epipolar line 502 belonging to the referencepoint. The difference angle 503 is determined between 501 and 502. Thisis done for each reference line point. Of all the determined angles, anaverage angle is then determined. When the difference angle approachesperpendicularity as for example 503 in FIG. 5, the projection isoptimal. The more the difference angle deviates from perpendicularity,the less optimal the projection is. This average directional differenceangle is then normalized.

In the case of a bifurcation, vessel tree or multiple single vessels,for each branch or vessel, the normalized average directional differenceangle is calculated. To obtain one normalized directional differenceangle for the projection, the normalized average directional differenceangles of each branch or vessel are weighed using a weighting function.A weighting function is a function that calculates the contribution of,in this case the normalized average directional difference angles, tothe total result based on for instance the diameter of the vessel.

An example of an optimal projection map for the directional differenceangle can be seen in FIG. 6. The x-axis of FIG. 6 depicts the rotationangles of the X-ray apparatus, while the y-axis of FIG. 6 depicts theangulation angles of the X-ray apparatus. For all combinations ofrotation and angulation angles of the X-ray apparatus, that is allpossible projections J, the normalized average directional differenceangle between projection J and projection I is calculated as describedabove.

In this example, all candidate projections resulting in a directionaldifference angle approaching perpendicularity to the first/primaryprojection I have been determined to be complementary (e.g. optimal),and are depicted white in the color map of FIG. 6, for example for theprojection J with an angulation of 30 and a rotation of 0. While allprojections resulting in a small directional difference angle aredetermined to be non-complementary (e.g. non-optimal) and are depictedblack in the color map of FIG. 6, for example for the projection J withan angulation of 35 and a rotation of −180.

To obtain one measure for how optimal a certain projection is, thenormalized spatial angle and the normalized directional difference angleare advantageously combined by the processor at 105 into one overallparameter through a weighting function. For example, for each possibleprojection the overall parameter has a value between 0 and 1 where 0 isleast optimal and 1 is most optimal. Obviously, each of the twoparameters can also be used separately.

The overall value of each possible projection is then shown in anoptimal projection map. The optimal projection map is a color map inwhich for each combination of rotation and angulation, that is for eachpossible projection, an overall value is shown using a correspondingcolor or grey value. An example of a color map is shown in FIG. 7. Inthis color map for example the outcome for the normalized spatial anglesof FIG. 4 is weighed with the outcome for the normalized directionaldifference angles of FIG. 6. This weighting function can for example bea multiplication. Reference 701 of FIG. 7 represent the normalizedspatial angle outcome of FIG. 4. Because these regions were indicatednon-complementary in terms of the normalized spatial angle, this is alsothe case for the overall parameter. Likewise, regions determined to benon-complementary in terms of directional difference angle (702) arealso indicated non-complementary for the overall parameter. Using thegenerated color map, the user can then select a projection that is mostoptimal relative to projection I. This projection can then be used toobtain the second two-dimensional angiographic image that can be usedfor further calculations, for instance generating a three-dimensionalreconstruction.

Optionally, another parameter regarding overlap can be considered.Because multiple organs, particularly vessels, are present in the x-raypath from x-ray source to detector, those organs are projected on theimage as well. Depending on the viewing perspective, these organs mayoverlap the object of interest. A certain projection is more optimal incase the overlap of surrounding vessels of the vascular system, forinstance the coronary tree, is minimal. An example of an embodimentwhere organ overlap is used as a parameter to help determine optimalityof the second projection is shown in FIG. 8.

For this overlap parameter, a 3D or 3D+time (3D+t) model of the vesseltree, for instance the coronary tree, is input at 702. The 3D model ofthe vessel tree can for instance be a generic centerline or lumen modelobtained by averaging various segmented CT or MRI datasets. A generic 3Dmodel can be available for each heart model (i.e. coronary dominantsystem).

When, for instance, a motion model is used to deform the 3D modelextracted from CT data a 3D+t model is available at 702, representingthe coronary morphology during the cardiac cycle. This can be done forinstance as taught by Baka et al, “3D+t/2D+t CTA-XA registration usingpopulation based motion estimates”, Demirci, Lee, Radeva, Unal (eds):MICCAI-STENT 2012, pp 64-71, where a method is proposed for buildingpopulation based average and predicted motion from 4D CT datasets whichis then used to perform 3D+t/2D+t registration based on distanceminimization on one cardiac cycle. The complete subject matter of thepublication referenced herein is incorporated by reference herein in itsentirety. The first angiographic image I is input at 701. Preferably theheart phase of the angiographic image is matched to that of the 3D or3D+t at 703. That is the 3D model represents one heart phase, whereasthe two-dimensional angiographic image contains multiple heart phases.Aligning the heart phase ensures a better matching. In the case of a3D+t model, the heart phase can be synchronized for instance using ECGeither acquired by digitalizing the ECG signal acquired from thepatients or retrieved by file transfer.

At 704, the processor determines how the 3D model corresponds to thetwo-dimensional angiographic image I. For this a rigid registration isperformed between the 3D model and the two-dimensional angiographicimage I as for instance taught by Guéziec et al, “Anatomy-BasedRegistration of CT-scan and Intraoperative X-ray Images for Guiding aSurgical Robot”, IEEE Transactions on Medical Imaging, Vol. 17, No. 5,October 1998. Guéziec et al teaches a registration method that computesthe best transformation between a set of lines in three space, the(intraoperative) X-ray paths, and a set of points on a surface. Thecomplete subject matter of the publication referenced herein isincorporated by reference herein in its entirety.

Once the 3D model and the two-dimensional angiographic image I have beenregistered, the processor can determine at 705 which possibleprojections J contain overlap of surrounding vessels.

For each possible viewing direction corresponding to a certainperspective, for instance a simulated X-ray beam from the image sourcetowards the section of interest in the 3D model, can be determined. Incase of vessel overlap, a surrounding vessel of the 3D model willintersect the X-ray-beam.

For each possible projection, the amount of overlap can thus bedetermined. The views that contain overlap of surrounding vessels areless optimal than views without overlap.

This overlap parameter can for instance be combined with the alreadycalculated parameters by adding the overlap parameter in the weightingfunction. This results in a color or grey level map that contains anoverall measure for optimality consisting of a spatial angle parameter,a directional difference angle parameter as well as an overlapparameter.

The embodiment described herein can be used on a standalone system orincluded directly in, for instance, an x-ray fluorographic system or anyother image system to acquire two dimensional angiographic images. FIG.9 illustrates an example of a high-level block diagram of an x-raycinefluorograpic system. In this block diagram the embodiment isincluded as an example how the embodiment could integrate in suchsystem.

Portions of the system (as defined by various functional blocks) may beimplemented with dedicated hardware, analog and/or digital circuitry,and/or one or more processors operating program instructions stored inmemory.

The X-ray system of FIG. 9 includes an X-ray tubes 801 with a highvoltage generator 802 that generates an X-ray beam 803.

The high voltage generator 802 controls and delivers power to the X-raytube 801. The high voltage generator 802 applies a high voltage acrossthe vacuum gap between the cathode and the rotating anode of the X-raytube 801.

Due to the voltage applied to the X-ray tube 801, electron transferoccurs from the cathode to the anode of the X-ray tube 801 resulting inX-ray photon-generating effect also called Bremsstrahlung. The generatedphotons form an X-ray beam 803 directed to the image detector 806.

An X-ray beam 803 consists of photons with a spectrum of energies thatrange up to a maximum determined by among others the voltage and currentsubmitted to the X-ray tube 801.

The X-ray beam 803 then passes through the patient 804 that lies on anadjustable table 805. The X-ray photons of the X-ray beam 803 penetratethe tissue of the patient to a varying degree. Different structures inthe patient 804 absorb different fractions of the radiation, modulatingthe beam intensity.

The modulated X-ray beam 803′ that exits from the patient 804 can bedetected by the image detector 806 that is located opposite of the X-raytube. This image detector 806 can either be an indirect or a directdetection system.

In case of an indirect detection system, the image detector 806 caninclude a vacuum tube (the X-ray image intensifier) that converts theX-ray exit beam 803′ into an amplified visible light image. Thisamplified visible light image is then transmitted to a visible lightimage receptor such as a digital video camera for image display andrecording. This results in a digital image signal.

In case of a direct detection system, the image detector 806 consists ofa flat panel detector. The flat panel detector directly converts theX-ray exit beam 803′ into a digital image signal.

The digital image signal resulting from the image detector 806 is passedthrough a digital image processing unit 807. The digital imageprocessing unit 807 converts the digital image signal from 806 into acorrected X-ray image (for instance inverted and/or contrast enhanced)in a standard image file format for instance DICOM. The corrected X-rayimage can then be stored on a hard drive 808.

Furthermore, the X-ray system of FIG. 9 consists of a C-arm 809. TheC-arm holds the X-ray tube 801 and the image detector 806 in such amanner that the patient 804 and the adjustable table 805 lie between theX-ray tube 801 and the image detector 806. The C-arm can be moved(rotated and angulated) to a desired position to acquire a certainprojection in a controlled manner using the C-arm control 810. The C-armcontrol allows for manual or automatic input for adjustment of the C-armin the desired position for the X-ray recording at a certain projection.

The X-ray system of FIG. 9 can either be a single plane or a bi-planeimaging system. In case of a bi-plane imaging system, multiple C-arms809 are present each consisting of an X-ray tube 801, an image detector806 and a C-arm control 810.

Additionally, the adjustable table 805 can be moved using the tablecontrol 811. The adjustable table 805 can be moved along the x, y and zaxis as well as tilted around a certain point.

A general unit 812 is also present in the X-ray system. This generalunit 812 can be used to interact with the C-arm control 810, the tablecontrol 811 and the digital image processing unit 807.

An embodiment is implemented by the X-ray system of FIG. 9 as follows. Aclinician or other user acquires an X-ray angiographic image of apatient 804 at a certain projection by using the C-arm control 810 tomove the C-arm 809 to a desired position relative to the patient 804.The patient 804 lies on the adjustable table 805 that has been moved bythe user to a certain position using the table control 811.

The X-ray image is then generated using the high voltage generator 802,the X-ray tube 801, the image detector 806 and the digital imageprocessing unit 807 as described above. This image is then stored on thehard drive 808. Using this X-ray image, the general processing unit 812calculates several parameters and provides the user with an optimalprojection map that contains for each possible angiographic projection ameasure for how optimal that projection would be as a second projection.

Using this outcome, the user can operate to acquire (or display) theimage that belongs to this optimal projection and continue the procedurewith the maximum amount of object information and the least amount oftime and burden to the patient spent on finding that information. Duringsuch operations, the general unit 812 can show rotation and angulationangles of the arm of the imaging system that correspond to the optimalprojection. The user can manually rotate the arm of the imaging systeminto a position that correspond to the chosen optimal projection or theC-arm control module 810 can automatically rotate the arm of the imagingsystem to the calculated optimal projection.

There have been described and illustrated herein several embodiments ofa method and apparatus for determining optimal image viewing directionin terms of reduced foreshortening and relevancy of information. Whileparticular embodiments have been described, it is not intended that theinvention be limited thereto, as it is intended that the invention be asbroad in scope as the art will allow and that the specification be readlikewise. For example, the data processing operations can be performedoffline on images stored in digital storage, such as a picture archivingand communication system (PACS) commonly used in the medical imagingarts. It will therefore be appreciated by those skilled in the art thatyet other modifications could be made to the provided embodimentswithout deviating from its spirit and scope as claimed.

The embodiments described herein may include a variety of data storesand other memory and storage media as discussed above. These can residein a variety of locations, for instance as on a storage medium local to(and/or resident in) one or more of the computers or remote from any orall of the computers across the network. In a particular set ofembodiments, the information may reside in a storage-area network(“SAN”) familiar to those skilled in the art. Similarly, any necessaryfiles for performing the functions attributed to the computers, serversor other network devices may be stored locally and/or remotely, asappropriate. Where a system includes computerized devices, each devicecan include hardware elements that may be electrically coupled via abus, the elements including, for example, at least one centralprocessing unit (“CPU” or “processor”), at least one input device (e.g.,a mouse, keyboard, controller, touch screen or keypad) and at least oneoutput device (e.g., a display device, printer or speaker). The systemmay also include one or more storage devices, for instance as diskdrives, optical storage devices and solid-state storage devices such asrandom access memory (“RAM”) or read-only memory (“ROM”), as well asremovable media devices, memory cards, flash cards, etc.

The devices also can include a computer-readable storage media reader, acommunications device (e.g., a modem, a network card (wireless orwired), an infrared communication device, etc.) and working memory asdescribed above. The computer-readable storage media reader can beconnected with, or configured to receive, a computer-readable storagemedium, representing remote, local, fixed and/or removable storagedevices as well as storage media for temporarily and/or more permanentlycontaining, storing, transmitting and retrieving computer-readableinformation. The system and various devices also typically will includea number of software applications, modules, services or other elementslocated within at least one working memory device, including anoperating system and application programs, such as a client applicationor web browser. It should be appreciated that alternate embodiments mayhave numerous variations from that described above. For example,customized hardware might also be used and/or particular elements mightbe implemented in hardware, software (including portable software, suchas applets) or both. Further, connection to other computing devices suchas network input/output devices may be employed.

Various embodiments may further include receiving, sending, or storinginstructions and/or data implemented in accordance with the foregoingdescription upon a computer-readable medium. Storage media and computerreadable media for containing code, or portions of code, can include anyappropriate media known or used in the art, including storage media andcommunication media, such as, but not limited to, volatile andnon-volatile, removable and non-removable media implemented in anymethod or technology for storage and/or transmission of information suchas computer readable instructions, data structures, program modules orother data, including RAM, ROM, Electrically Erasable ProgrammableRead-Only Memory (“EEPROM”), flash memory or other memory technology,Compact Disc Read-Only Memory (“CD-ROM”), digital versatile disk (DVD)or other optical storage, magnetic cassettes, magnetic tape, magneticdisk storage or other magnetic storage devices or any other medium whichcan be used to store the desired information and which can be accessedby the system device. Based on the disclosure and teachings providedherein, a person of ordinary skill in the art will appreciate other waysand/or methods to implement the various embodiments.

The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. It will, however, beevident that various modifications and changes may be made thereuntowithout departing from the broader spirit and scope of the invention asset forth in the claims.

Other variations are within the spirit of the present disclosure. Thus,while the disclosed techniques are susceptible to various modificationsand alternative constructions, certain illustrated embodiments thereofare shown in the drawings and have been described above in detail. Itshould be understood, however, that there is no intention to limit theinvention to the specific form or forms disclosed, but on the contrary,the intention is to cover all modifications, alternative constructionsand equivalents falling within the spirit and scope of the invention, asdefined in the appended claims.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the disclosed embodiments (especially in thecontext of the following claims) are to be construed to cover both thesingular and the plural, unless otherwise indicated herein or clearlycontradicted by context. The terms “comprising,” “having,” “including”and “containing” are to be construed as open-ended terms (i.e., meaning“including, but not limited to,”) unless otherwise noted. The term“connected,” when unmodified and referring to physical connections, isto be construed as partly or wholly contained within, attached to orjoined together, even if there is something intervening. Recitation ofranges of values herein are merely intended to serve as a shorthandmethod of referring individually to each separate value falling withinthe range, unless otherwise indicated herein and each separate value isincorporated into the specification as if it were individually recitedherein. The use of the term “set” (e.g., “a set of items”) or “subset”unless otherwise noted or contradicted by context, is to be construed asa nonempty collection comprising one or more members. Further, unlessotherwise noted or contradicted by context, the term “subset” of acorresponding set does not necessarily denote a proper subset of thecorresponding set, but the subset and the corresponding set may beequal.

Operations of processes described herein can be performed in anysuitable order unless otherwise indicated herein or otherwise clearlycontradicted by context. Processes described herein (or variationsand/or combinations thereof) may be performed under the control of oneor more computer systems configured with executable instructions and maybe implemented as code (e.g., executable instructions, one or morecomputer programs or one or more applications) executing collectively onone or more processors, by hardware or combinations thereof. The codemay be stored on a computer-readable storage medium, for example, in theform of a computer program comprising a plurality of instructionsexecutable by one or more processors. The computer-readable storagemedium may be non-transitory.

Preferred embodiments of this disclosure are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ the variations asappropriate and the inventors intend for embodiments of the presentdisclosure to be practiced otherwise than as specifically describedherein. Accordingly, the scope of the present disclosure includes allmodifications and equivalents of the subject matter recited in theclaims appended hereto as permitted by applicable law. Moreover, anycombination of the above-described elements in all possible variationsthereof is encompassed by the scope of the present disclosure unlessotherwise indicated herein or otherwise clearly contradicted by context.

All references, including publications, patent applications and patents,cited herein are hereby incorporated by reference to the same extent asif each reference were individually and specifically indicated to beincorporated by reference and were set forth in its entirety herein.

The invention claimed is:
 1. A computer implemented method for guidingselection or acquisition of projection images of an object, the methodcomprising: a) providing a first two-dimensional image of the objectwhich has been acquired from a first perspective orientation withreference to a coordinate system; b) defining at least one spatial angleparameter in the reference coordinate system that is indicative of adegree to which any candidate perspective orientation in the referencecoordinate system complements the first perspective orientation; c)calculating values of the at least one spatial angle parameter of b) fora plurality of candidate perspective orientations; d) evaluating thevalues of the at least one spatial angle parameter for the plurality ofcandidate perspective orientations as calculated in c) to select oneparticular candidate perspective orientation that complements the firstperspective orientation; and e) obtaining or acquiring a secondtwo-dimensional image of the object which has been acquired from the oneparticular candidate perspective orientation as selected in d).
 2. Themethod according to claim 1, wherein: the evaluation of d) involvesdisplaying a map that includes axes that represent coordinates of thecoordinate system and indicia indicative of the values of the at leastone spatial angle parameter at varying coordinates of the coordinatesystem, wherein the indicia represent a color or grey value that isassociated with combinations of the coordinates.
 3. The method accordingto claim 2, wherein: the first perspective orientation and the pluralityof candidate perspective orientations are expressed in the coordinatesystem and in a form of rotation and angulation of an x-ray machine thatis configured to acquire the first two-dimensional image and the secondtwo-dimensional image.
 4. The method according to claim 1, wherein: thevalues of at least one spatial angle parameter for the plurality ofcandidate perspective orientations as calculated in c) are based on atleast one of: i) a three-dimensional angle between the first perspectiveorientation and a respective candidate perspective orientation, ii) adirectional difference angle between a reference line in the firsttwo-dimensional image and an epipolar line that corresponds to arespective candidate perspective orientation and that is projected onthe first two-dimensional image, and iii) at least one value thatrepresents degree of overlap between organs.
 5. A method according toclaim 1, wherein: the at least one spatial angle parameter comprises adirectional difference angle between a reference line in the firsttwo-dimensional image and an epipolar line that corresponds to arespective candidate perspective orientation and that is projected onthe first two-dimensional image.
 6. The method according to claim 5,wherein: the object is a tubular organ or comprises a region containingtubular organs, the reference line being the centerline of the tubularorgan or organs.
 7. The method according to claim 6, wherein: the objectcomprises a plurality of tubular organs, the reference line is thecenterline of at least part of the tubular organs, and the evaluating ofd) employs a weighting function based on the direction difference anglesfor different parts of the plurality of tubular organs.
 8. The methodaccording to claim 1, wherein: the at least one spatial angle parameteris represented by a value assigned to a three-dimensional angle betweenthe first perspective orientation and a respective candidate perspectiveorientation, wherein the value assigned to a three-dimensional angle inthe range at or less than 30° and at or higher than 150° is a minimumvalue corresponding to a least optimal perspective.
 9. The methodaccording to claim 1, wherein: the object comprises a plurality oforgans; the at least one spatial angle parameter is based on at leastone value that represents degree of overlap between organs; and a 3D or3D+t model of the plurality of organs is built or input for use indetermining values that represent different degrees of overlap betweenorgans to obtain corresponding projection images.
 10. The methodaccording to claim 9, wherein: a registration is performed between thefirst two-dimensional image and the 3D or 3D+t model before determiningthe values that represent different degrees of overlap between organs.11. The method according to claim 10, wherein: the 3D model is a 3D+tmodel; and two-dimensional angiographic images containing multiple heartphases are synchronized with the heart phases of the 3D+t model using anECG signal before registration.
 12. The method according to claim 9,wherein: the candidate perspective orientations containing organ overlapare assigned values that correspond to a least optimal perspective,while the candidate perspective orientations containing little or nooverlap are assigned values that correspond to a most optimalperspective.
 13. The method according to claim 1, wherein: angulationand rotation angles of the plurality of candidate perspectiveorientations are limited within a range of possible rotation andangulation angles of an apparatus used for acquiring the first andsecond two-dimensional images.
 14. A system for guiding selection oracquisition of projection images of an object, the system comprising: atleast one processor; and a memory coupled to the at least one processor,wherein the memory stores program instructions, wherein the programinstructions are executable by the at least one processor to: a) providea first two-dimensional image of the object which has been acquired froma first perspective orientation with reference to a coordinate system;b) define at least one spatial angle parameter in the referencecoordinate system that is indicative of a degree to which any candidateperspective orientation in the reference coordinate system complementsthe first perspective orientation; c) calculate values of the least onespatial angle parameter of b) for a plurality of candidate perspectiveorientations; d) evaluate the values of the at least one spatial angleparameter for the plurality of candidate perspective orientations ascalculated in c) to select one particular candidate perspectiveorientation that complements the first perspective orientation; and e)obtain or acquire a second two-dimensional image of the object which hasbeen acquired from the one particular candidate perspective orientationas selected in d).
 15. The system according to claim 14, wherein: theevaluation of d) involves displaying a map that includes axes thatrepresent coordinates of the coordinate system and indicia indicative ofthe values of the at least one spatial angle parameter at varyingcoordinates of the coordinate system, wherein the indicia represent acolor or grey value that is associated with combinations of thecoordinates.
 16. The system according to claim 15, wherein: the firstperspective orientation and the plurality of candidate perspectiveorientations are expressed in the coordinate system and in a form ofrotation and angulation of an x-ray machine that is configured toacquire the first two-dimensional image and the second two-dimensionalimage.
 17. The system according to claim 14, wherein: the object is atubular organ or comprises a region containing tubular organs; and theat least one processor is configured to identify a reference linerepresenting a centerline of the tubular organ or organs.
 18. The systemaccording to claim 14, wherein: the at least one spatial angle parameteris represented by a value assigned to a three-dimensional angle betweenthe first perspective orientation and a respective candidate perspectiveorientation, wherein the value assigned to a three-dimensional angle inthe range at or less than 30° and at or higher than 150° is a minimumvalue corresponding to a least optimal perspective.
 19. The systemaccording to claim 14, wherein: the candidate perspective orientationsthat contain organ overlap are assigned non-complementary values for thespatial parameter, while the candidate perspective orientations thatcontain little or no overlap are assigned a complementary values for thespatial parameter.
 20. The system according to claim 14, wherein: thevalues of at least one spatial angle parameter for the plurality ofcandidate perspective orientations as calculated in c) are based on atleast one of i) a three-dimensional angle between the first perspectiveorientation and a respective candidate perspective orientation, ii) adirectional difference angle between a reference line in the firsttwo-dimensional image and an epipolar line that corresponds to arespective candidate perspective orientation and that is projected onthe first two-dimensional image, and iii) at least one value thatrepresents degree of overlap between organs.
 21. The system according toclaim 14, further comprising: an x-ray machine configured to acquire thefirst two-dimensional image.
 22. The system according to claim 21,wherein: the x-ray machine is further configured to acquire the secondtwo-dimensional image.
 23. The system according to claim 14, furthercomprising: an angiographic apparatus with X-ray source and imageintensifier located at opposite sides of an arm, the arm being movableat least according to a rotation angle and an angulation angle withreference to a patient to obtain two-dimensional images from differentperspectives, the at least one processor including a data processingmodule being programmed to calculate rotation and angulation angles ofthe arm for obtaining optimal projection images.
 24. The systemaccording to claim 22, wherein: the angiographic apparatus is configuredto automatically or semi-automatically rotate the arm, or display userindications for manually rotating the arm, according to rotation andangulation angles calculated for obtaining an optimal projection image.25. A non-transitory computer-readable storage medium having storedthereon executable instructions that, when executed by one or moreprocessors of a computer system, cause the computer system to: a)provide a first two-dimensional image of the object which has beenacquired from a first perspective orientation with reference to acoordinate system; b) define at least one spatial angle parameter in thereference coordinate system that is indicative of a degree to which anycandidate perspective orientation in the reference coordinate systemcomplements the first perspective orientation; c) calculate values ofthe least one spatial angle parameter of b) for a plurality of candidateperspective orientations; d) evaluate the values of the at least onespatial angle parameter for the plurality of candidate perspectiveorientations as calculated in c) to select one particular candidateperspective orientation that complements the first perspectiveorientation; and e) obtain or acquire a second two-dimensional image ofthe object which has been acquired from the one particular candidateperspective orientation as selected in d).
 26. The non-transitorycomputer-readable storage medium according to claim 25, wherein: theevaluation of d) involves displaying a map that includes axes thatrepresent coordinates of the coordinate system and indicia indicative ofthe values of the at least one spatial angle parameter at varyingcoordinates of the coordinate system, wherein the indicia represent acolor or grey value that is associated with combinations of thecoordinates.
 27. The non-transitory computer-readable storage mediumaccording to claim 25, wherein: the first perspective orientation andthe plurality of candidate perspective orientations are expressed in thecoordinate system and in a form of rotation and angulation of an x-raymachine that is configured to acquire the first two-dimensional imageand the second two-dimensional image.